I did edit some of the ads after their original posting, but it was only to edit the TEXT in the body of the ad, I didn't (and couldn't) touch the price or anything related to the price. Did you try editing your ad (I have HTML WYSIWYG editor turned ON) and see if it does anything to the RESLIMIT? Of course if RESLIMIT gets changed to Zero while the ad is still active and then it ends, any auction having at least 1 bid is going to close as sold because it is going to think you ran an auction without a reserve.
The only custom code we have for our classifieds is for adding new adds in a certain category (non-auction, not this area of the classifieds) from an XML feed into the database. That only adds new car ads, it doesn't ever touch existing ads and doesn't even do anything to the reslimit field because the car ads are all fixed price, never an auction.
Try turning on your WYSIWYG editor and then making some edits to an auction ad and saving it and see what happens. Eventually we'll find the culprit!